Beyond the data model, OPC UA defines how clients and servers actually communicate — sessions, subscriptions, discovery, and the intervals that control how often data updates. This section also covers newer communication models like OPC UA PubSub and OPC UA FX.

Term Definition
Session A logical, authenticated connection between an OPC UA client and server that persists across one or more underlying network connections.
Subscription A standing request from a client that causes a server to monitor specified data and events and report changes at a defined interval.
Monitored Item An individual Node, attribute, or event source tracked within a Subscription.
Publishing Interval The rate at which a server sends queued data change and event notifications to a client for a given Subscription.
Sampling Interval The rate at which a server checks a Monitored Item’s underlying value for changes.
Deadband A defined threshold of value change required before a server reports an update, used to reduce unnecessary network traffic.
Discovery The process by which an OPC UA client finds available servers and their connection details on a network.
Discovery Server (LDS) A Local Discovery Server that maintains a registry of OPC UA servers running on a host, allowing clients to find them.
Polling A communication method where a client periodically requests the current value of data from a server, rather than waiting for the server to push changes.
Tunneling / Tunneler A technique that carries OPC Classic (DCOM-based) traffic across TCP/IP, avoiding DCOM’s firewall and network configuration limitations.
OPC UA PubSub An OPC UA communication model where servers publish data to a message broker or network and any number of subscribers can consume it, rather than relying on direct client-server connections.
OPC UA FX (Field eXchange) An OPC UA extension aimed at extending the standard down to field-level devices, enabling controller-to-controller and device-level communication.